SEDIMENTATION CONTROL MODERN TECHNIQUES

DAMS SEDIMENT CONTROL SYSTEMS www.hammadjatt.weebly.com Engr.Hammad Bashir
HAMMAD BASHIR 11-C-809
2013
USER1
11-C-809
Submitted To:-
PROF,Muhammad
Shafqat Mehboob
[SEDIMENTATION CONTROL MODERN TECHNIQUES]

SEDIMENTATION CONTROL MODERN
TECHNIQUES
SEDIMENT CONTROL:
Sediment and erosion control is a critical necessity to ensure
protection of fish and other aquatic species when undertaking any maintenance activity.
Sediment in a watercourse can disrupt many of the life processes of fish and other aquatic life.
Sediment related impacts can include:
• Injury or death. Sediment can clog or abrade fish gills causing suffocation or tissue damage.
• Inability to feed. Since fish are visual feeders, sediment in water can impair their ability to forage
for food.
• Infill habitat. Sediment may settle in pools or riffle habitats. This may impact salmonid egg
viability or may affect food and nutrient sources by covering benthos (bottom-dwelling organisms)
upon which fish depend for food.
• Increased wear on mechanical equipment or other machinery. Sediment can also cause plugging
of irrigation intake screens.

Modern Techniques to control sedimentation:
Surface Erosion Controls:
1. Mulching or Mats:
Spreading straw or hay mulch over exposed soils may be an
effective temporary measure to control erosion. Mulch can help reduce the erosive energy of rainfall
such that agricultural soil, manure or sediment is not transported to a watercourse. Since some
mulch can suppress the regeneration or growth of desired plants, they should only be used when
recommended by a professional in an overall plan. Where large areas of exposed soils are present on
unstable slopes, coco-matting may be considered. This may also be a technique used when works
are completed late in the season if seed not established.

2. Silt Fencing:
Silt fencing may be an appropriate sediment control mechanism
after drainage maintenance works are completed. It is generally recommended that ditch spoil is
placed above the top of bank where its re-entry to a watercourse will not occur. If it is placed where
its entry to a watercourse is possible then silt fending should be considered in addition to seeding.
When installing a silt fence, the bottom of the filter material must be trenched into the ground, See
Figure 3 for details. After a rain event inspect the silt fence. A build up of 30 cm or more should be
removed from the edge of the fence.

Temporary Instream Sediment Controls for Low Flows
Outlet Blocking:
 An easy method of sediment control is to block the ditch outlet
using plywood or a ‘plug’ of clean material (e.g. gravel) at the outlet.
Anything used to block an outlet must be removed once the work is
completed, and should not be in place from November to June of
any year. For removal of the temporary blockage, re-release of
water into the channel should be done gradually, and only once
sediment has settled out of the water and the water is clear.
 Check dams are structures that provide a small barrier to water
flow, behind which water is temporarily ponded allowing sediment
to settle out.
 Check dams can also be used to isolate small work areas and to
prevent sediment laden water from being transported downstream.
Check dams should only be used in ditches that have small amounts
of ponded water very low flows.
Figure Temporary Check Dam

Isolation and Diversion Techniques for Instream Work
When working in a ditch that has flowing water, the work area should be isolated and the water
diverted around the work area. Isolation of water flow may be undertaken using several different
methods. The method chosen should be appropriate for the amount of flowing water in the
watercourse.
Several accepted methods for water flow isolation are described below.
• Irrigation gates or other instream structures.
• Blocking an instream culvert.
• Constructing a coffer dams
In all cases a temporary diversion channel must be constructed that routes the flow around the work
site.
 Cofferdams:
Cofferdams are used to isolate a section of watercourse from flowing water.
Materials for cofferdams should be selected based on ease of maintenance and complete removal
following construction activities.
The dam may be made of:
 Rock
 Sand bags
 Wood
 Sheet metal
 Gravel or earthen plug
If water is flowing in the ditch, cofferdams may be used in combination with other methods such as
clean water bypasses and/or pumps.
Wood or sheet metal dams must be anchored into the banks of the ditch to prevent seepage and
erosion around the edges of the dam, see Figure
During construction, inspect the dam. Extra material (for example, more sandbags) should be kept
on site to raise the dam if water behind the dam threatens to spill over into the work area.
Immediately repair any gaps, holes or scour around the dam to prevent failure.
Upon completion of maintenance works, the structure should be removed gradually to prevent
sediments from moving downstream or erosion from occurring.

Isolation Techniques for Instream Works

 Channel Diversions:
Channel diversions are used where it is possible to redirect
the water flow temporarily around the section of the ditch where work is being conducted. A stream
diversion is a temporary bypass through a pipe or a flume may be used if there is enough grade for
gravity flow. An excavated channel is also an option, although it is more labor intensive and can
generate large amounts of sediment. See Figure for examples of channel diversions.
FIGURE Stream Diversion Techniques

 Pumped Diversion:
The pumped diversion is suitable for intermittent and low
flow streams that can be pumped. Pump capacity must be sufficient to handle the flow in the ditch.
Temporary dams are constructed upstream and downstream of the work area.
Advantages of a pumped diversion include:
• Downstream sediment transport can almost be eliminated.
• De-watering of the work area is possible.
• Pipes can be moved about to allow construction operations.
• Increased flows can be managed by adding more pumping capacity.
Some Disadvantages of a pumped diversion include:
• Flow volume is limited by pump capacity.
• Requires 24-hour monitoring of pumps
. • Sudden rain could overtop dams.
• Minor in-stream disturbance to install and remove dams.
• Erosion at pipe outlet.
Figure Typical Pumped Stream Diversions

Pumping Over Temporary Clay Dam

 Bypass Flumes:
A bypass flume can be constructed out of a heavy pipe that is large
enough to handle the water flow in the watercourse. See Figure for an example of a bypass flume.
Advantages of excavated channels and flumes are:
• Isolates work from water flow and allow dewatering
• Can handle larger flows than pumps
Disadvantages of excavated channels and flumes are:
• Channels must be protected from erosion
• Flow diversion and then re-direction with small dams causes instream disturbance and sediment.
In stream silt fences should be used if sedimentation is occurring. See Figure.
Figure Temporary Silt Fence

SEDIMENTS REMOVAL BY DEVICES:
The process is set up by devices allowing continuous and controlled transfer of sediment within the
reservoir in a relatively small scale but permanent mode. Key element is an automatically working
vessel with a suction dredging system installed that can be diesel or preferably electric driven. To
allow for an exact positioning the vessel is directed by tractor cables. Sediments are loosened by a
suction head, pumped towards the reservoir’s outlet and dumped in front of the outflow elements.
The vessel gradually strikes the reservoir floor until the complete surplus sediment is removed.
Sediment transfer vessel “SediMover” (automatic, electrically driven)
The newly dumped sediments are eroded by the hydraulic discharge and therefore carried out of the
reservoir, passing turbines or outlet valves. Both types of outflow elements are able to handle water
flow with some degree of sediment load. The sediment transfer rate can be adapted to the outflow
rate and parameters to guarantee a compatible process speed. For an environmentally sound
project implementation the time span should be set to months or years, depending on local
conditions. A commercially competitive project performance is still assured by a high degree of
automation.
Continuous sediment transfer (ConSed-Method)

SEDIMENTATION CONTROL MODERN TECHNIQUES

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